Biogenic oxidation of water-soluble metal ions into insoluble oxides has been taking place for millions of years leaving its signature all around us. Nature’s evident success in producing nanophase metal oxides is inspiring scientists to understand and learn from these processes.

The creation of nanostructured electrode materials for energy storage may represent one of the most attractive strategies and a path forward to dramatically improving the performance of both Li-ion and beyond Li-ion systems. Short diffusion length associated with the nanoscale dimensions effectively reduces the distance that ions and electrons must travel during cycling relative to the equivalent bulk material. Enhanced kinetics significantly improves capacity and rate capability, suppression of phase transformations improves electrochemical reversibility, and the use of defects and high surface area produces high capacity for intercalation. Furthermore, nanomaterials can offer a possible solution to excellent electronic and/or ionic conductivity required for unhindered charge flow with their ability to connect materials and build up structures from the molecular level.

Electrochemical synthesis is a facile way for preparation of nanoscale architectures. Utilization of electrochemical deposition brings a high level of control to the structure, morphology, and uniformity of electrodes by adjusting the crucial parameters such as applied current, potential, electric pulses, as well as the temperature and concentration of the electrolyte. Most of the known conventional bulk battery materials may start out as highly crystalline materials but upon many repeated cycles can pulverize overtime leading to non-electrochemically active, and electrically disconnected particles. Our previous work has shown that by starting from amorphous, low-crystalline materials by electrochemical cycling we can create nanostructured electrodes with self-optimized crystalline phases (cycling of TiO₂ amorphous nanotubes in Li batteries converted them into cubic titania¹). Self-organization of materials during electrochemical cycling could be an easy general approach for synthesis of new, optimized crystalline forms of materials for energy applications.

Manganese oxide-based materials are an ideal platform for understanding the underlying properties that determine capacity for a broad spectrum of energy storage chemistries due to the availability of multiple valence states for charge storage and the relative ease and low cost with which they can be synthesized. MnO₂ exists in various crystallographic polymorphs, namely α-, ß-, γ, δ-, λ-, and ε-type. These highly crystalline, structurally disordered or amorphous phases show unique electrochemical activity as a result of complex factors such as: structural intergrowths, lattice defects, cation vacancies, random octahedral-unit (MnO₆) distribution, proton diffusion, conductivity within the oxide particles.

In order to accelerate the implementation of nanostructured systems in beyond Li-ion batteries functional links must be developed between nanoscale materials structure and the resulting performance characteristics. My recent results showed that it is possible to extend reversible ion-insertion chemistry of MnO₂ from monovalent Li to ions with higher charge: Zn ²⁺. Starting from low-crystalline layered manganese oxide upon electrochemical cycling new phase was formed at the nanoscale². One of the most important prerequisites to understanding this process is the knowledge of the atomic-scale structure of its product. Furthermore, combined with a metal anode, understanding the underlying intercalation mechanism of these systems in a multivalent metal cell and the underlying principles behind reversible intercalation of multivalent ions into a cathode (e.g., intercalation vs conversion and possible structural transformation compared to an aqueous system) is a valuable asset for multivalent cell design.

Developing new synthetic electrochemical techniques to better control the nature and structure of nanostructured oxide materials and modify environment of layered oxide materials to tune ability to host various ions will ultimately enable the design of optimal structures that can operate with multiple transporting ions with higher capacity and dramatically improved cycle life.

Acknowledgement This work was supported by the U. S. Department of Energy, US DOE-BES, under Contract No. DE-AC02-06CH11357.

References

1. Self-Improving Anode for Lithium-Ion Batteries Based on Amorphous to Cubic Phase Transition in TiO2 Nanotubes Hui Xiong, Handan Yildirim, Elena V. Shevchenko, Vitali B. Prakapenka, Bonil Koo, Michael D. Slater, Mahalingam Balasubramanian, Subramanian K. R. S. Sankaranarayanan, Jeffrey P. Greeley, Sanja Tepavcevic, Nada M. Dimitrijevic, Paul Podsiadlo, Christopher S. Johnson, and Tijana Rajh The Journal of Physical Chemistry C 2012 116 (4), 3181-3187

2. Mechanism of Zn Insertion into Nanostructured δ-MnO2: A Nonaqueous Rechargeable Zn Metal Battery Sang-Don Han, Soojeong Kim, Dongguo Li, Valeri Petkov, Hyun Deog Yoo, Patrick J. Phillips, Hao Wang, Jae Jin Kim, Karren L. More, Baris Key, Robert F. Klie, Jordi Cabana, Vojislav R. Stamenkovic, Timothy T. Fister, Nenad M. Markovic, Anthony K. Burrell, Sanja Tepavcevic, and John T. Vaughey Chemistry of Materials 2017 29 (11), 4874-4884